In most cases, if I look for a fiction book and it is not by an author known to me, just by reading a page or two I can judge if book could be worth reading or not. Of course, no one can make fully qualified review by reading just two pages (although many do
), but in most cases I can make a choice narrower without missing a lot.
Non-fiction is another story. Here you can base mostly on reviews and, although they are usually correct, they are no guarantee at all. At the moment I'm reading a non-fiction book that made lot of buzz but I think that 3/4 of it is absolutely not applicable in majority of cases. That makes it not valuable for me and pretty much loss of time.